The present invention relates to a method for the molecular weight fractionation of polyhydrogen silsesquioxane. More specifically, the present invention relates to a highly reproducible method for the molecular weight fractionation of polyhydrogen silsesquioxane that yields a very storage-stable polyhydrogen silsesquioxane with any desired molecular weight.
Polyhydrogen silsesquioxane is a ladder-type or cage-type hydrogen-substituted polysiloxane with the following general formula: EQU (HSiO.sub.3/2).sub.n
in which n is an integer. By utilizing the reactivity of the Si-bonded hydrogen present in the molecule, a variety of organofunctional groups can be introduced into polyhydrogen silsesquioxane or the polyhydrogen silsesquioxane can itself be bonded to substrates such as organic resins and so forth. Moreover, since the polyhydrogen silsesquioxane molecule is free of organic groups, the silica afforded by its self-condensation characteristically has excellent heat resistance and excellent dielectric properties.
Polyhydrogen silsesquioxane is typically produced by the hydrolysis and polycondensation of trichlorosilane or trialkoxysilane (refer to Japanese Patent Publication Number Sho 47-31838 [31,838/1972] and Japanese Patent Application Laid Open [Kokai or Unexamined] Numbers Sho 59-189126 [189,126/1984], Sho 60-42426 [42,426/1985], and Sho 60-86017 [86,017/1985]). The polyhydrogen silsesquioxane products of such processes are mixtures whose molecular weight distribution in each case ranges from several hundreds to several hundred thousands. In addition, the products of such processes contain large amounts of residues from the reaction catalyst used during synthesis and they contain ionic impurities in the form of alkali ions eluted from the reactor, heavy metal ions, and so forth.
When polyhydrogen silsesquioxane with a broad molecular weight distribution is used for the formation of silica thin films in the field of microelectronics (for example, as an interlevel dielectric in semiconductor elements), problems such as cracking in the silica thin film and so forth occur. In addition, polyhydrogen silsesquioxane which contains impurities, e.g., ionic impurities, etc., cannot even be used in microelectronic applications. For these reasons, fractionation of polyhydrogen silsesquioxane into a desired molecular weight distribution and reduction in its level of impurities (e.g., ionic impurities, etc.) is required.
In one example of a method for the molecular weight fractionation of an organosilsesquioxane, Japanese Patent Application Laid Open Number Sho 61-127732 [127,732/1986] proposes the molecular weight fractionation of an organosilsesquioxane by the addition of an organic solvent whose solubility limit corresponds to the maximum average molecular weight and an organic solvent whose solubility limit corresponds to she minimum average molecular weight to be fractionated. However, the high reactivity of the silicon-bonded hydrogen in polyhydrogen silsesquioxane precludes the use of certain solvents such as alcohols, amines, and mercaptans in the molecular weight fractionation method taught in this reference. Moreover, application of this method is hampered by the different solubility behaviors of organopolysilsesquioxane and polyhydrogen silsesquioxane in various solvents. Finally, the method taught in this reference employs large quantities of several solvents. As a result, molecular weight fractionation by this method is poorly reproducible, inefficient and the yield of polyhydrogen silsesquioxane product is substantially reduced.
It is also generally known to purify and remove ionic impurities from organopolysiloxanes by washing with water. However, washing polyhydrogen silsesquioxane with water results in gelation due to the high reactivity of the silicon-bonded hydrogen in polyhydrogen silsesquioxane.
The object of the present invention is the introduction of a highly reproducible method for the molecular weight fractionation of polyhydrogen silsesquioxane that yields a very storage-stable polyhydrogen silsesquioxane which has a desirable molecular weight.